One of the characteristics of the Industrial Revolution has been a shift from small scale batch (craft like) operations to large scale continuous (plant based) mass production. Ford’s automotive assembly line is the typical example of mass production, but the same kinds of changes also occurred in the chemical industry. Labor intensive batch distillation was replaced with continuous distillation which allowed a much greater chemical throughput. Just as mass production techniques greatly reduced the price of a “Model T” automobile, it also greatly reduced the price of the gasoline which powered that machine.

There are two major types of continuous distillation columns, but both operate in basically the same way. In both cases liquid is continuously fed into the column, and at least two streams (distillate and bottoms), together containing the same amount of total material, are continuously removed. Heat is added to the re-boiler (pot) and removed at the condenser. The re-boiler vaporizes some of the liquid, which then follows a treacherous path to the top of the column where it is re-condensed. Along the way most of the high boiling compounds will been left behind, and the distillate will be quite pure. To further aid the separation process some of the liquid distillate is often returned to the column where it flows back to the bottom. Along the way this reflux condenses some of the higher boiling liquids out of the vapor phase helping to purify the vapor. The two types of columns are:

Tray Columns (shown above): Such columns consist of physically separated pools of liquid which are in intimate contact with a vapor. Bubble columns are often used to force the upward flowing vapor through these pools of downward flowing liquid. Each of these trays operates as an equilibrium stage (like the pot and water examples above).

Packed Columns: Such columns are filled with a saddle shaped packing that resembles Styrofoam peanuts. This packing provides a lot of surface area for the vapor to condense upon and assures that the liquid and vapor are in intimate contact.

Whereas the composition of the distillate and bottoms in batch distillation changes over time, a continuous column operates under steady conditions where the composition at a given location does not change over time. This steady state operation is desired in almost all continuous unit operations. Because the composition only depends upon the position in the column, additional product steams can be easily tapped at different heights (not shown) and each tray will have a different composition of compounds. The trays at the top of the column are rich in light boiling compounds while those at the bottom are rich in compounds that only boil at high temperatures.